WASHINGTON - U.S. drug reviewers questioned findings from biotech company Allos Therapeutics Inc (ALTH.O) on the effectiveness of its proposed drug for an aggressive type of lymphoma, documents released on Friday showed.

Allos shares fell 6.2 percent to $7.29 in afternoon trading on Nasdaq. The company is counting on the product, Folotyn, to be its first drug to reach the market.

Food and Drug Administration reviewers, in a summary prepared for an advisory panel, said their two issues with Allos' application were the clinical significance of tumor response and duration of response as well as whether the benefit:risk ratio is favorable for Folotyn.

Allos is seeking approval to sell the drug for previously treated patients with peripheral T-cell lymphoma (PTCL). There are no approved therapies for the disease.

The FDA reviewers said they would ask an advisory panel that meets on Wednesday for input on those issues.

Wells Fargo analyst Aaron Reames, in a research note, said he remained positive on the prospects for a favorable panel vote based on previous support for a cancer drug with similar data, a robust response rate in a highly desperate patient population and other factors.

He said it was a significant positive sign that there was no language suggesting a concern with ... Folotyn's safety profile in the FDA staff summary.

Allos, in a separate summary, said PTCL patients who have failed to respond to other therapies were in desperate need of better treatment options. Folotyn offers these patients a therapy demonstrated to induce clinically meaningful responses, the company said.

Allos reported that 29 patients, or 27 percent, responded to treatment with Folotyn. FDA reviewers, however, said the agency cannot verify these responses and their duration in these 13 responders except that these responses lasted (less than) 14 weeks.

The input from the advisory panel will be key as the agency usually follows panel recommendations. A final decision on whether to approve the drug is due by Sept. 24, although the agency has missed several such deadlines in recent months.

The drug's generic name is pralatrexate.

The FDA and Allos documents were posted here. (Reporting by Lisa Richwine, editing by Gerald E. McCormick and Tim Dobbyn)